The Ethernet ring system is a ring consisting of several nodes connecting with one and another, and several domains, that is, the Ethernet ring protection domains may be defined on the ring. One domain comprises one master node and several transit nodes and a group of protected service Virtual Local Area Networks (VLAN) which are used to forward the services of the user data and a control VLAN which is used to forward the protocol messages, wherein, the two ports of the master node on the ring are defined as the primary port and the secondary port respectively, and the service VLAN which is protected is defined as the protected service VLAN. When the link works well, the master node disables the secondary port's capability of forwarding protected service VLAN, and there is no loop formed in the network, avoiding the “broadcasting storm” due to the loop; when the link fails, the secondary port's capability of forwarding the protected service VLAN is enabled so that the user data can pass through the secondary port of the master node to guarantee the connectivity of the service.
FIG. 1 is a topological graph of the Ethernet ring automatic protection system which consists of node S1, S2, S3 and S4, and an Ethernet ring protection domain is configured on the ring consisting of these four nodes, the master node in the domain is S2, and the rest are the transit nodes. The two ports of S2 on the ring are the primary port and the secondary port respectively, wherein port 2 is the primary port (P), and port 1 is the secondary port (S). In addition, the control VLAN and the protected service VLAN are defined in the domain, with each port on the ring configured in the control VLAN and the protected service VLAN. When the link on the ring works well, the master mode S2 disables the secondary port 1's capability of forwarding the protected service VLAN data to avoid forming loop path in the network which may result in the “network storm”; when the link fails, the master node S2 enables the secondary port 1's capability of forwarding the protected service VLAN data to make the data in the service VLAN be re-connected.
The control messages in the Ethernet ring automatic protection system mainly comprises the health detection message of HELLO, the link failure alarm message of LINK-DOWN, a link failure notification message of FLUSH-DOWN, and a link recovery notification message of FLUSH-UP. The communication and decision making among the nodes in the Ethernet ring protection system are performed by these messages.
In one domain, the master node sends the health detection message of HELLO out periodically through the primary port, and the HELLO message is transmitted in the control VLAN of the domain, and when the link on the ring works well, the HELLO message arrives at the secondary port of the master node through the transit nodes one by one. When the master node receives the HELLO message from the secondary port, it considers that the link works well; and when the master node does not receive the HELLO message in the due time, it considers that the link fails.
When a transit node detects that the status of the port associated with the directly connected link on the ring changes from working well to failure, it sends the LINK-DOWN message to the master node which acquires that the link on the ring fails after receiving the message.
When the master node finds out that the HELLO message is expired or receives the LINK-DOWN message, it considers that the link on the ring fails, then enables the secondary port's capability of forwarding the protected service VLAN and sends the FLUSH-DOWN message to notify the other nodes that the link on the ring fails and they should update the Media Access Control (MAC) address table of the ports of the nodes on the ring.
After a transit node receives the FLUSH-DOWN message, it acquires that the link on the ring fails and then updates the MAC address table of its ports on the ring.
When the link fails, the master node still sends the HELLO message out periodically from the primary port, and if the master node receives the HELLO message from the secondary port, it considers that the link is recovered and then disables the capability of forwarding protected service VLAN of the secondary port, and sends the FLUSH-UP message to the transit nodes to notify them that the link is recovered.
After a transit node receives the FLUSH-UP message, it acquires that the failure of the link on the ring has been resolved and then updates the MAC address table of its ports on the ring.
In the Ethernet ring automatic protection system, the transmission of the HELLO message on the ring is the most frequent. In order to shorten the convergence duration of the link switch in the Ethernet ring automatic protection system, the transmission period of the HELLO message is usually set to be relatively small, some times smaller than one second, and accordingly the transmission frequency of the HELLO message is relatively high. In addition, when there are multiple Ethernet protection domains on the same physical ring, each domain will send out its own HELLO message. Therefore, each node on the ring will receive a lot of HELLO messages. However, among the nodes of the Ethernet ring automatic protection system, only the master node's processor is required to process the HELLO message while the transit nodes just need to transmit the HELLO message rather than process it with their processors, therefore, when the processors of the transit nodes receive the HELLO message periodically, the processing by the processor is invalid. In the control messages of the Ethernet ring automatic protection system, only the FLUSH-DOWN and FLUSH-UP message need to be processed by the processors of the transit nodes, and when other messages are sent to deteriorate the reliability of the system and greatly waste the system resource of the transit nodes.